The Sun might look like a perfect sphere at sunset, but it isn’t. As stars (planets too) rotate, they are flattened by centrifugal force. It’s not always noticeable to the human eye, but scientists can measure it.
Because the Sun rotates with a period of 27 days, its radius at the equator is slightly larger than at the poles. 10 kilometers (6.2 miles) larger. For the Earth, the radius difference is even larger at 21 kilometers (13 miles).
Researchers use a technique called asteroseismology to determine exactly how round a star is. And they applied this technique to a star located 5,000 light-years away. Kepler 11145123. It’s twice the size of our Sun but rotates three times slower. By studying the oscillation of Kepler 11145123 for a long-period of time, researchers can nail down the difference in radius between the equator and poles.
For 4+ years, NASA’s Kepler continuously watched the star. Led by Laurent Gizon (Max Planck Institute, University of Göttingen), researchers compared the frequencies of the modes of oscillation that are more sensitive close to the equator with those more sensitive at poles.
Here’s what they found. The radius at the equator is just 3 kilometers larger than the poles with a margin of error of 1 kilometer. “This makes Kepler 11145123 the roundest natural object ever measured, even more round than the sun,” says Gizon.
Gizon and his team believe a magnetic field at low latitudes could also be helping the star look more spherical.
Kepler 11145123 sits in the record books for the roundest object right now, but it might not last. Researchers plan to take the same method used for observing this star and applying it to others seen by Kepler. And for future missions including TESS (launch expected in December 2017) and PLATO (launch expected in 2024).
The researchers are particularly interested in seeing how stronger magnetic fields could shape a star with a faster rotation rate. “An important theoretical field in astrophysics has no become observational,” says Gizon.